The problem with the blinkies is that you don't know where the camera manufacturers have set the limit? I think it is wrong to assume it is 255 255 255 or 100%. I believe it is a lower value but they won't tell.

Whether it's 245 or 255 is of no concern because such a difference would represent about 1/6th of a stop in exposure...you're still going to drop exposure by 1/3 stop either way. Besides...it's the RAW data that we're concerned about most...what's most important is that you understand the relationship between the RAW data and the RGB image that is the basis for the histogram (which is what UniWB is all about.)

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Are you serious when you say that you are concerned about the lightness values in a small area such as a person's eyes?

Have you ever taking portrait shots? Clip the eyes or the teeth and it's noticeable...it looks like there's something wrong with the image (and there is.)

Much as I'm enjoying this discussion, can I just remind you all that this is the Beginners forum - I imagine that the OP will be sitting under a table, clutching his knees to his chest and rocking gently back and forth by now..!

I must confess that I fail to see why "18%" or "12.7%" would ever have to be mentioned in a discussion about digital cameras. Film cameras might be another story, but I don't know much about those.

The gray reference is still important to digital cameras because that's how the camera meter works.

Lets look back several posts. I made the statement that if there's any white in the scene, then you can't ETTR. I was asked to explain that statement. The explanation is that if you set standard exposure...that is, set your exposure by spot metering a 12.7% gray reference, then any objects that are white will be near saturation. Therefore, you can't ETTR ("ETTR" taken to mean pushing exposure beyond standard exposure for the purpose of increasing signal.)

If your scene consist entirely of mid-tones, then you can ETTR, increase signal, and reduce noise. But the brighter the highlights in the scene, the closer you'll get to standard exposure because your overexposure latitude keeps shrinking as the highlights get brighter.

Yes, ETTR is a sound concept. Just have to realize that on scenes with white objects and bright highlights, standard exposure is pretty much the same as an exposure arrived at through ETTR.

.that is, set your exposure by spot metering a 12.7% gray reference, then any objects that are white will be near saturation.

you are assuming that amount of light falling on two does not influence the outcome... you might gave a grey object illuminated so that it will reflect more light in the end than white object... plus 12.7% is how spot meter assumes the reflectance of the target (and it varies - you need to check how your camera model meters, even around 12.7% is how most of them do), not actual reflectance of the gray reference.

you are assuming that amount of light falling on two does not influence the outcome... you might gave a grey object illuminated so that it will reflect more light in the end than white object... plus 12.7% is how spot meter assumes the reflectance of the target (and it varies - you need to check how your camera model meters, even around 12.7% is how most of them do), not actual reflectance of the gray reference.

You're creating random situations, and that's of no help. Exposure is based on the light source. When you have a single light source, setting exposure by spot metering a 12.7% card (or equivalent, compensated reference such as an 18% card with +0.5 EC applied) will give you an exposure where white (90%+ reflectance) will be near saturation...always.

When a scene has two light sources then you have to start making compromises. That's what you have in outdoor scenes where part of the frame is lit by direct sunlight and the other part is in open shadow (the sky glow is the light source, which is why light in open shade is so blue.) In such a case, you have to decide what's important and adjust exposure accordingly. But just because such situations exist doesn't mean we toss the basic knowledge. A gray card is just a tool...but a valuable one if you understand how to use it.

And the 12.7% doesn't vary. Otherwise, you couldn't use a handheld meter to set exposure.

There's a youtube video of some guy attempting to demonstrate how to calibrate a Canon 7D to a Sekonic meter. It's terrible. But although everything he did was wrong, the video indirectly demonstates that the 7D, Sekonic, and another Minolta meter that he used were all calibrated exactly the same way (before he ruined the Sekonic with his "calibration".) Nikon is also 12.7% gray.

I keep hearing that exposure varies but I never see it demonstrated. On the other hand, I know that if you spot meter a white or gray wall, the metered area will be rendered as sRGB 100 (before any processing) on Canon, Nikon, and Pentax at least (and Sekonic meters will give the same exposure values.)

Zone system metering was not part of the original brief but is a valid approach to metering.In a lot of outdoor situations a similar exposure to ETTR will result because of the high dynamic range of the scene.

If you adjust exposure so as to minimize the headroom above the brightest parts of the scene, you are moving the histogram such that it almost touches the clipping point. Some call this ETTR, but I think that it is counter-productive. The procedure should be to move those parts of the scene that you _care_ about as far as possible to the right. This could mean clipping highlights if you only want them approximately recorded (or plan to crop them later on). The point is to maximize the recorded signal level without clipping, assuming that higher recording levels means higher SNR. We might call this ETTR or whatever, I might call it "sensible digital recording levels", or SDRL :-)

An exposure strategy based on placing the middle part of the histogram (or some other particular quantile) at any particular place along the camera DR is fundamentally different. It seems suited for a camera technology that have symmetric error mechanisms in highlights and shadows. It _might_ happen to give similar exposure values as ETTR for some scene/camera combinations, but that is more of a coincidence.

http://www.cambridgeincolour.com/tutorials/camera-metering.htm"What constitutes middle gray? In the printing industry it is standardized as the ink density which reflects 18% of incident light, however cameras seldom adhere to this. This topic deserves a discussion of its own, but for the purposes of this tutorial simply know that each camera has a default somewhere in the middle gray tones (~10-18% reflectance). Metering off of a subject which reflects more or less light than this may cause your camera's metering algorithm to go awry — either through under or over-exposure, respectively....many cameras contain sophisticated algorithms which try to circumvent this limitation, and estimate how bright an image should be. These estimates frequently result in an image whose average brightness is placed in the midtones. "

First, most people don't realize that Adams' Zone System included film development and print processing in order to produce an image of Adams' vision of the scene...which may or may not (more often not) match human vision of the scene. First you meter the different areas of the scene to get the luminance differences, then you decide what you want those differences to actually be, and where you want them on the EV scale. Finally you expose and develop and print to realize your vision. The Zone systems is more about producing and image than capturing an image.

Regardless...you still need to understand how the meter works, and that's all that I explained. Spot meter a 12.7% gray surface, and white objects are near saturation. That's how it works.

You answered the question I posed. The reason I posed it was that it is indeed a different method.

<The Zone systems is more about producing and image than capturing an image.>

I don't see why you differentiate between producing and capturing? It is surely the end product that counts? Assuming that there isn't a "correct" way of getting an exposure and only a subjective way then what ever it takes is surely valid? The zone system uses lower EC as well as higher EC which means that it is pertinent to the OP,s question.

"This topic deserves a discussion of its own, but for the purposes of this tutorial simply know that each camera has a default somewhere in the middle gray tones (~10-18% reflectance)."

No source for this information is listed. As I said, Canon, Nikon, Pentax cameras are calibrated to 12.7% (sRGB 100, 100, 100), and Sekonic meters are supposedly 12.5% (sRGB 99, 99, 99) according to their website. I can't speak for all cameras, but if a camera meter is calibrated to ISO standards then it should turn out to be 12.7%.

I'm not differentiating production and capture generally...only as related to the application of the Zone System in its classic form. The Zone system was applied through the entire process, whereas ETTR is applied at capture time. The resulting capture should allow you to accomplish your pictorial goals, but the goal of the process called ETTR is achieved entirely at capture time. I was simply drawing a distinction.

I think you are confusing film thinking with digital thinking. The original zone system was perfected for film and later adapted for digital. The digital way of doing things is implemented at capture. A previous poster also pointed out that the mid tone way of doing exposure is more suited to film than digital. A lot of photographers who make the leap from film to digital still try to use a mid tone and some a light meter. They then struggle to implement the thinking to adapt to digital and struggle.

I'm not sure what I'm supposed to get from that. After presenting the same unsubstantiated information that everyone else does and making some quesitonable presumptions, it seems like he took a few pictures and then proceeded to draw conclusions that probably should have been experimentally verified with an additional set of captures demonstrating saturation at the predicted overexposure levels.